1. Field of the Art
The present invention relates to a magneto-optical memory medium which is formed with a plurality of magneto-optical data storage layers, to improve the density of information stored per unit area of the medium. The invention also relates to a magneto-optical apparatus for writing information on a selected one of the storage layers of such a magneto-optical memory medium, and a magneto-optical apparatus for reading information from the selected storage layer.
2. Related Art Statement
There is known a magneto-optical recording and reproduction apparatus for recording or writing information on a magneto-optical material which has a self-magnetizing property and through which a beam of light may be transmitted. The information is written by means of local magnetization of the magneto-optical material in one of opposite directions normal to the surface of the magneto-optical material. The written information on the magneto-optical memory medium is read out or reproduced by utilizing a magneto-optical effect. A commonly used magneto-optical memory medium for information storage comprises a thin data-storage layer of a magneto-optical material formed on a substrate layer made of an acrylic resin, glass or other suitable material. A magneto-optical medium of such a laminar structure in the form of a disk is generally known as a magneto-optical disk. The data storage capacity of such a known magneto-optical memory medium, i.e., the number of data bits recordable on the memory medium, is determined by the size of a spot of a light beam used for information recording or reproduction. Since the size of the beam spot is closely related to the wavelength of the light beam, the number of data bits per unit area of the memory medium is limited by the wavelength of the beam of light used. Thus, the density of information stored on the known memory medium has a limitation in relation to the wavelength of the light beam. In the meantime, a magneto-optical memory medium has a limitation in its size, due to increasing tendencies of deformation of its substrate layer and dimensional errors of the medium as the size is increased. For example, the maximum permissible diameter of a magneto-optical disk is 30 cm. As indicated above, the known magneto-optical memory medium suffers a limitation in its memory capacity.